Improving Sensitivity of Solution ³¹P NMR Analysis in Australian Xeralfs

Abstract

Phosphorus-31 nuclear magnetic resonance (³¹P NMR) spectroscopy is widely used to identify and quantify phosphorus (P) forms in soil. This study aimed to determine whether narrowing the soil to extractant sodium hydroxide–ethylenediaminetetraacetic acid (NaOH-EDTA) ratio from 1:20 to values as low as 1:4 would improve sensitivity of solution ³¹P NMR spectroscopy without degrading resolution or quantitation. Four Australian soils were tested using four ratios. The narrowest ratio of 1:4 gave the best quality NMR spectra in terms of signal-to-noise ratio. Peak resolution was not degraded on narrowing the ratio. There was no clear effect of narrowing the extraction ratio on extraction efficiency or the distribution of signal among chemical shift regions (orthophosphate, monoester P, diester P, and pyrophosphate). We conclude that a ratio of 1:4 improved NMR analysis for these particular soils and should be considered for other soils, particularly low-P soils, where NMR sensitivity is limiting.

Keywords:

• ³¹P nuclear magnetic resonance spectroscopy
• soil phosphorus
• soil to extractant ratio

Additional information

Funding

We thank CSIRO for providing soil samples collected as part of the National Soil Carbon Research Program, funded by the Australian Government Department of Agriculture, Fisheries, and Forestry, the Grains Research and Development Corporation, and the Department of Environment, Water, and Natural Resources. The first author gratefully acknowledges funding from a DIKTI-Indonesia scholarship.